Laser-cooled microgravity clocks

The principle advantage of microgravity for atomic clocks is interrogation times longer than 1 s. With a 10 s interrogation time, a clock has a 50 mHz linewidth suggesting that accuracies may potentially exceed 10/sup -16/. However, to achieve greater accuracy within the same averaging time, greater stability is needed. Achieving greater stability in a microgravity clock constrains the design differently than for earth based fountains. In this paper, we discuss the design considerations for laser-cooled microgravity clocks highlighting the considerations that differ from those for earth-based fountains. As in earthbased fountains, the frequency shift due to cold collisions plays an important role in the design of the clock. Given our predictions (and measurements) for the shift in laser-cooled Rb clocks, we currently anticipate building a high performance Rb clock and discuss the relative merits of Rb and Cs microgravity clocks. Finally, we present our tentative designs for two microgravity clocks.